loader: Add scenarii around new visit with changes since last visit
In this scenario, a visit has already taken place for the origin. So, an existing snapshot exists.
But, this time, the PyPI project has changed and a new release has been uploaded.
The visit must then result in a new snapshot.
The snapshot shares the same history as prior visit.
Related swh-loader-core#421 (closed)
To be rebased and merged after olasd's diffs (done once).
Test Plan
make test
Migrated from D416 (view on Phabricator)